1. Field of the Invention
This invention relates to a semiconductor memory device with MOS transistors, each including a floating gate and a control gate, and to a memory card including the semiconductor memory device. More particularly, this invention relates to a nonvolatile semiconductor memory device, such as a flash memory.
2. Description of the Related Art
Various types of flash memories have been proposed for a variety of uses.
For instance, a NAND flash memory has been known as a data storage memory for use in a digital camera or the like. A NAND flash memory has been described in, for example, Imamiya K., et al., “A 125-mm/sup 2/1-Gb NAND Flash Memory With 10-Mbytes/s Program Speed,” IEEE Journal of Solid-State Circuits, Vol. 37, No. 11, November, 2002, pp. 1493–1501.
Another known flash memory is a NOR flash memory which writes and erases data by using FN tunneling. A NOR flash memory has one memory cell transistor and two select transistors (hereinafter, referred to as a 3Tr-NAND flash memory). A 3Tr-NAND flash memory has been written in, for example, Ditewig T., et al., “An Embedded 1.2-V Read Flash Memory Module in a 0.18-μm Logic Process,” Solid-State Circuits Conference, 2001 Digest of Technical Papers ISSCC. 2001 IEEE International 5–7, February, 2001, pp. 34–35, 425.
Furthermore, in recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed. A flash memory of this type has memory cells, each including one memory cell transistor and one select transistor (hereinafter, referred to as a 2Tr flash memory). A 2Tr flash memory has been disclosed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997.
With the recent progress in the processing technology, the memory cells are being miniaturized more in the above flash memories. However, it has been difficult to reduce the size of the peripheral circuits around the memory cell array, particularly the size of the decoder circuits.